Don't skimp on the cayenne salt rim—it adds a kick to the grapefruit flavor.
Mixer: In a large jar, bottle, or pitcher, stir 4 tsp. agave syrup and 4 tsp. hot water until blended. Let cool. Stir in 1 cup blanco tequila. Cover and refrigerate until cold, about 2 hours or up to 1 week. Up to 2 hours before serving, stir in 1 1/4 cups fresh pink grapefruit juice and 3 tbsp. fresh lime juice and keep refrigerated. Stir well before using.
Garnish: On a small plate, stir 2 tbsp. kosher salt and 1/4 tsp. ground cayenne. Rub a lime wedge around the rim of each Champagne flute, then dip the rim in the cayenne salt.
Cocktail: In each prepared flute, add 1/4 cup of the mixer. Carefully top with chilled dry sparkling wine (such as Cava). Stir gently.